Corrosion protection for metals



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INVENTOR. STANLEY L CH ISHOLM 8. Qlfiw ATTORNEY United States Patent 3,311,529 CORROSION PROTECTION FOR METALS Stanley L. Chisholm, 2503 Commonwealth Ave., San Diego, Calif. 92104 Filed May 23, 1963, Ser. No. 289,153 4 Claims. (Cl. 161-213) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties thereon or therefor.

The present invention relates to the treatment of metallic surfaces against corrosion, and more particularly to new and improved compositions, means and methods of preventing and controlling corrosion through thenovel use of the metalloids antimony and arsenic and certain compounds of these metalloids. This application is a continuation-in-part of my patent application Ser. No. 134,- 483, filed Aug. 28, 1961, now abandoned.

The problem of corrosion of metals, including alloys, is greatly magnified in \maringgrvi-ronments, The use of more susceptible metals having desirable properties is limited or prevented because of the inadequacy of present means and methods of controlling their corrosion in such an environment. An outstanding example is that of magnesium alloys, the common structural metal most difiicult to protect in a saline environment. The light weight and comparatively great strength of magnesium makes this metal very suitable for use in airplanes, helicopters, boats and many naval devices. However, its use in marine environments has been severely restricted because of lack of adequate corrosion protection. Where it has been used, maintenance has been onerous; the Navy has a continuous problem to keep aircraft containing magnesium alloy components in operational status at sea.

The means presently employed to protect magnesium and the high strength aluminum alloys includes the use of inhibitive primer formulations containing low solubility chromates, molybdates, vanadates, selenates, and fluorides such as those of zinc, strontium, calcium, barium, lithium and cadmium. While these may serve the purpose in nonsaline environments, they are much less effective under saline conditions. The protective polarization of such primers is lost in water with salinity as low as of that of the ocean. Air-dry lacquer systems such as those incorporating acrylic or acrylic-nitro-cellulose resins are similarly inadequate in marine environments. Baked enamel coatings such as. those of siliconeepoxy-alkyd resin formulations, are considerably more effective but they cannot be utilized for many applications of said alloys because the baking temperatures cannot be tolerated by the metal, or by the assembled article, or because they cannot be readily replaced when worn off or accidentally scratched.

Steel is protected against corrosion by the use of the same primer formulations, containing chromate and molybdate inhibitors, as mentioned above. This generally useful system requires frequent renewal, especially at liquid-air interfaces and in saline environments. The electroplating of steel and iron with other metals, including antimony and arsenic, protects the steel or iron, but it is ditficult to renew such protection because an externally generated electromotive force must be used.

3,311,529 Patented Mar. 28, 1967 "ice None of the priming or plating protective systems presently employed are very effective where chemically dissimilar structure metals are coupled or bonded together. Under such circumstances, galvanic corrosion is usually far more rapid and destructive than the corrosion caused by the environment alone. Of the commonly used structural metals, the most severe deterioration occurs when magnesium alloys are coupled with steel or iron. Such coupled magnesium, even where conventionally painted, disintegrates at a rate at least several hundred times that of uncoupled magnesium in the same saline environments. There are countless applications where the combination of steel and magnesium is desirable, e.g., a steel screw or attachment fastener on a magnesium alloy airplane component. Plating of the steel is an improvement over the coupling of bare steel, but does not give adequate protection because the conventional plating metals such as zinc or cadmium themselves similarly accelerate the corrosion of magnesium. This is especially true in a saline or marine environment. The same problem exists, though to a lesser degree, where high strength aluminum alloys are coupled with steel or plated steel. Conventional inhibitive primers such as zinc chromate primer prove to be inadequate when such chemically dissimilar metals are coupled in a marine environment. Prior art does not adequately solve the corrosion problem actually experienced by the foregoing coupled dissimilar metals in a saline or marine environment.

This invention provides corrosion-resistant protective coatings for metals which embrace all the advantages of similarly employed corrosion-inhibiting systems and possess none of the aforescribed disadvantages. To attain this, the present invention contemplates the utilization of the corrosion-inhibitive properties of the metalloids antimony and arsenic, and certain low solubility compounds of these metalloids. For the protection of metals, such as light metals, whose rate of corrosion would normally be increased in the presence of these metalloids and compounds, the application of a barrier layer between the metalloid and the metal to be protected is essential, unless the metalloid is combined with an anionic corrosion inhibitor in sufficient proportions in a single layer. At the same time, the effectiveness of the well known chromate or molybdate corrosion inhibitors is greatly enhanced, if such an inhibitor is added or incorporated into the barrier layer. Thus, the metalloid hearing layer and the conventional barrier layer can be separate layers or they may be combined in a single layer, as will later be described more fully. Galvanic corrosion caused by the coupling or bonding of dissimilar metals is sufficiently controlled so that the use of desirable structural metals need not be limited by inability to prevent such corrosion. Where one of the coupled metals is antimonyor arsenic-plated steel, the invention causes the metalloid to directly protect not only the steel, but indirectly 'and more particularly also the metal with which the steel is coupled. Equipment need not be disassembled in order to renew protection against corrosion, for this invention provides a means for achieving such protection that is as simple as applying a coat of paint. The composition, means and methods of the invention give metals better protection against corrosion than heretofore made possible.

An object of the present invention is to provide means for giving a metal greater protection against corrosion than heretofore possible.

Another object is to provide methods for utilizing the metalloids antimony and arsenic and nonhalide com pounds thereof for the protection of other metals, not only iron and steel.

A further object of the invention is to provide the means of corrosion protection in readily usable form, so that neither special equipment nor disassemblage of the metal parts to be protected is necessary for renewal of said protection.

Still another object is to provide a means of preventing and controlling galvanic corrosion between dissimilar metals, such as light metals and heavy metals so that the most suitable combination of structural metals can be utilized for a given purpose without limitations presently imposed because of inability to control such corrosion.

Yet another object of the present invention is to provide a means for insulating a metal member from corrosive reaction with the metalloids antimony and arsenic and nonhalide compounds thereof, and at the same time affording said metal member the benefit of the passivation or corrosion-resistant properties of said metalloids toward external corrosive agents.

A still further object is to provide a means for making easily corroded metals, such as aluminum and magnesium alloys, suitable for more widespread use in marine and other saline environments.

Another object is to provide a synergist to make more effective the chromate, molybdate, selenate and vanadate corrosion inhibitors used in paints and primers.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same become better understood by reference to the following detailed description, when considered in connection with the accompanying drawing in which like reference numerals designate like materials throughout the figures thereof and wherein:

FIG. 1 is a diagrammatic sectional view showing application of the invention to any metal, whether ferrous or nonferrous.

FIG. 2 is a diagrammatic sectional view showing a modification of the invention applied to metals such as iron or steel that do not react adversely with the metalloids antimony and arsenic or the nonhalide compounds thereof, or showing a modification where the metal is a light metal and the protective coating of the metalloid contains a substantial proportion of a conventional anionic inhibitor.

FIG. 3 is a diagrammatic sectional view showing a modification of the invention applied to antimonyor arsenic-plated steel or iron.

FIG. 4 is a diagrammatic vertical sectional view showing the specific embodiment of the invention, e.g., the dissimilar metal combination such as that of a steel or plated steel rivet inserted into a dissimilar metal skin as of an aluminum or magnesium alloy of an airplane, illustrating the application of the invention to couples between any dissimilar structural metals, whether ferrous or nonferrous.

FIG. 5 is a diagrammatic vertical sectional view showing a modified embodiment of the invention, e.g., a steel rivet inserted into the magnesium alloy skin of an air plane, illustrating the application of the invention to couples between ferrous and dissimilar structural metals.

Referring now to the drawing, wherein ike reference characters designate like or corresponding parts throughout the several views, FIG. 1 illustrates the general application of the invention to any metal 11, ferrous or nonferrous, in need of protection from corrosion. Under marine conditions all except the most noble metals tend to corrode. The corrosion products so formed, even where they are of such a nature as to tend to protect the basic metal; when they can directly contact or otherwise react with one of the light alloys of aluminum or magnesium, will greatly accelerate the corrosive attack and consequent disintegration of the light alloy. The so-called high strength, light metal alloys, such as aluminum, magnesium, and titanium are particularly susceptible to this attack as very greatly accelerated by the oxides of dissimilar base meals, whether my direct contact or by the saline promoted migration of their dissolution products to the light alloy surface. Such bivalent metals as cadmium or zinc when plated on iron or steel in order to improve the protection of the latter are relatively ineffective in preventing accelerated attack of the light alloy components, because of such ionic migration of dissolved oxide, presumably as bicarbonate.

This adverse reaction is prevented by utilizing one of the metalloids of this invention, the controlled solubility of which under marine conditions can be utilized to passivate a base heavy metal, such as iron or steel, and reduce or control its corrosion, and at the same time can be prevented from reacting directly and adversely with the light alloy surface by the interposition of a simple resinous or paint film barrier, which may be either unpigmented, with any compound which is not deleterious to the light alloy surface, such as a metal chromate. Such a barrier may be any one of the very numerous formulations which are commonly employed in the protection of light alloys, such as an acrylic or other resinous lacquer or an epoxy or other enamel, with or without inert mineral or inorganic pigmentation. The effectiveness of the method described herein extends not only to the heavy metals, i.e. iron, carbon or stainless steel, brass, or copper alloys (which it may be desirable to employ as attachment fittings or adjacent to light alloy structures, such as those of alumin um, magnesium or titanium) but, also, to those heavy metals which are plated with conventional electroplated coatings, such as those of cadmium. zinc, or nickel which also produce undesirable corrosive effects when employed in conjunction with the mentioned light alloys. Thus, one application of the invention is the combination of a barrier layer 13, which is applied directly to the metal 11, and a protective coating \15, including a metalloid selected from the group consisting of antimony, arsenic, and the low soluble nonhalide compounds thereof which is hereby defined as a metalloid constituent, dispersed in an conventional paint or primer vehicle, and applied in a conventional manner. By the term low solubility metalloid compound is meant that its solubility in water, or in the corrosive marine medium or condensate, preferably is about 3 to 30 and does not exceed about 300 parts per million by weight in order to insure reasonably continued chemical activity and integrity of the paint coating.

As will be later described with reference to FIG. 2, if the metal 11 is a heavy metal, the barrier layer 13 may be eliminated.

Halide compounds are generally unsatisfactory because they are too soluble and because such compounds react with the organic vehicles usually used for paint-type coating compositions, an acid being formed by the reaction. This cannot be tolerated in a protective coating system. Neutralization of the acid and subsequent washing of the composition would have to be extremely closely controlled to ensure the required chemical neutrality of said composition. Such a cumbersome procedure is unnecessary where suitable nonhalides are used, as taught by this invention.

The metalloid compound should be of limited solubility which may be defined as a low solubility in order to function effectively and persistently as a paint additive without impairing the integrity of the protective paint coat 15. Oxides, trioxides, sulfides and sulfoxides of antimony and arsenic are examples of suitable metalloid compounds. Antimony is somewhat preferred over arsenic because it is less toxic and, therefore, safer to utilize in the manufacture and application of the protective coating composition.

The corrosive action of these metalloids upon widely used nonferrous structural metals is the reason that their use as corrosion inhibitors has been limited chiefly to iron and steel, with which they do not react adversely. Even in distilled water the rate of corrosion of magnesium in contact with antimony or arsenic is 70% greater than that of magnesium alone in the same environment. The rate of corrosion is accelerated still more under marine or saline conditions. A significant disclosure of this invention is that the corrosion-resistant properties of these metalloids can now be utilized to protect such metals by separating the metalloid from the metal by use of a physical barrier 13, or by its combination with a conventional inhibitor, such as zinc chromate in a single layer. In effect, the physical barrier 13, where employed, functions to insulate the metal member from the direct corrosive action of the metalloid while the metalloid forms a barrier to outside corrosive agents. It appears that products of the slow corrosion of the metalloid particles or solution of its compounds in protective coat form passive unilateral or semiconductive chemical barriers at the heavy metal surface which suitably restrict the passage of ionic corrosive agents through the physical barrier 13 to the metal 11.

Examples of a protective coating 15 which have been employed are as follows:

TABLE I.LACQUER PRIMER Total solids by weight Antimony trioxide (percent of total solids) 47-53 Siliceous extender (talc) (percent of total solids) 53-47 Non-volatile vehicle content by weight (42%):

RS /2-second nitrocellulose (percent of n.v.

vehicle) 23-27 Alkyd resin (percent of n.v. vehicle) 73-77 Volatile content (23%):

Esters and ketone (percent of volatiles) 30 (min) Alchols (percent of volatiles) 1522 Hydrocarbons (percent of volatiles) (max.)

An actual performance control formula is as follows:

Control formula Ingredients: Weight in grams Antimony trioxide 702 Extender pigment (very fine talc) 120 Alkyd resin Xylol) 880 Maleic anhydride 2.5 Xylol 463 10% aluminum stearate gel 93 50% nitrocellulose resin solution /2 second) 225 Lead naphthenate drier 10 Cobalt naphthenate drier (6% metal) 2.5 Anti skinning agent 2.5

The foregoing paint is normally reduced with two parts of toluol to spraying consistency to permit the uniform application of a coating with a dry film thickness of 0.0003 to 0.004 inch.

TABLE II.EPOXY PRIMER Component 1: Weight percent Antimony trioxide 10.0 Magnesium silicate 6.5

6 For the above formulation the epoxy equivalent for the epoxy resin (solid) should be 425 to 550, for the polyamide resin (solid) the amine value 210 to 220.

Barrier layer 13 may be one of a wide variety of conventionally acceptable paints or primer formulations and it may be brushed, sprayed or rolled on to the metal 11, or dip coated. The paints which may be so employed include nitrocellulose, acrylic, silicone, polyester, vinyl;

cellulose acetate or butyrate or other lacquers or various natural or artificial gums, such as alkyld or epoxy if of the varnish or enamel type. These may be employed clear or non-pigmented, or suitably pigmented. Pigmented finishes more frequently produce the most effective barrier action, such as presently available conventional primer formulations either of lacquer or enamel, pigmented with a considerable proportion of Zinc or strontium chromate. The results obtained from combining these primer or barrier paint formulations with the synergistic effects of the metalloid constitutent, or adjunct, of the invention are quite generally decidedly superior to those obtainable by the employment of these coatings as presently conventionally applied in the art alone. The thickness and number of layers of barrier 13 is not particularly critical. It is important to understand that the sole purpose of barrier layer 13 in this use is to separate metal 11 from the metalloid in protective coat 15, Where the barrier layer alone does not serve adequately as protection against either environmental corrosion or galvanic corrosion engendered by the coupling or bonding of the metal 11 to a dissimilar metal. As heretofore mentioned, the metalloid in protective coat 15 provides the passivation corrosion protection to the metal 11. This system of protection could not be applied to widely used light nonferrous metals but for the effective performance of the barrier layer 13. One limitation in the selection of the material of barrier layer 13 is that it be chemically inert or protective to any metal which it may contact. Such protection is effected for example by the incorporation of a chromate or molybdate pigment, as will later be described.

The amount of metalloid incorporated into the protective coat 15 is variable according to the severity of the corrosive conditions. Under severe conditions it has been found that a weight of metalloid equal to the weight of the pigment ordinarily used in conventional paints gives excellent results.

The particle size of the finely-divided metalloid is limited only by the requirement that it be dispersable throughout the vehicle.

In the one application of the invention, i.e., the combination of a metalloid and a barrier layer, provides corrosion protection to all conventional structural metals, such as the ferrous and cuprous heavy metal alloys and the light metal alloys of magnesium, aluminum and titanium in saline, marine, fresh water and atmospheric environments. The application of a decorative paint finish on top of the protective coat 15 will in no way impair the effectiveness of this invention.

Instead of using a chemically-inert composition, a preferred barrier layer 13 comprises a conventional corrosion-resistant paint or primer in which the corrosion inhibitor is a low soluble chromate, such as zinc yellow or. strontium chromate, barium or strontium molybdate, selenate or vanadate. The products of the slow corrosion of the metalloid in the protective coat 15 exert a synergistic effect upon the conventional chromate or molybdate inhibitor so that the combination of the metalloid and the inhibitor furnishes far greater protection against corrosion than could be expected from the combination individually. Laboratory experiments indicate that the solubility of the inhibitor is increased, thereby increasing its corrosion-inhibiting activity localized at the immediate metal-paint boundary. The same solubility range of 3 to 300 parts per million parts of water applies to the pigments as apply to the solubility of the metalloid compounds previously described.

Where the metal 17 to be protected is found to be chemically compatible with metalloids, such as the heavy metals, as is illustrated in FIG. 2, a barrier layer 13 need not be applied, although it does not impair the efficiency of the invention to apply such a layer. Iron, steel, copper, and brass are examples of aforesaid metals that do not react adversely with the metalloids. The protective coat 15, comprising finely-divided metalloid carried in a vehicle, is all that is required to protect ferrous metals or other heavy metals from corrosion to an extent fully equal to that obtained by plating with antimony or arsenic. Tests with arsenic-plated steel coupled to magnesium alloys in a fresh-water environment reveal that at the start the steel starts to rust and its electropotential rises 50%. Soon thereafter the corrosion of the steel halts and the electropotential drops, suggesting that arsenic itself does not prevent corrosion of steel, but rather that products of the slow corrosion of said arsenic form protective barriers. This would explain why a metalloid in finely-divided form, with more surfaces exposed to corrosion, affords steel as much protection from corrosion as a solid-plated layer of metalloid. Incorporation of the metalloid into a paint or primer provides an ease of application that makes this means of protecting iron and steel from corrosion superior to the well-known system of continuously plating said ferrous metals.

A preferred protective coat 15 is a paint or primer wherein the finely-divided metalloid is added to a conventional corrosion-resistant paint or primer in which the inhibitor is a slightly soluble chromate, molybdate, selenate or vanadate. The synergistic effect of the metalloid on the inhibitor gives more corrosion protection than heretofore possible. Depending upon the severity of the corrosion conditions, an amount of metalloid constituent ranging substantially up to three times the weight of the conventional inhibitor is added. This protective coating 15 can be applied directly to the ferrous metal 17 to be protected (FIG. 2), as distinguished from the case of the previously described widely-used nonferrous metals, where a barrier layer 13 must be utilized.

The specific requirements of any successful primer coating for direct application to bare or chemically treated metal surfaces with so-called chemical conversion coatings includes good and persistant adhesion to the metal or surface treated substrate and good adhesion without adverse lifting of prime coats when top coats, as required, are applied. These requirements apply also to the protective or barrier coating of this invention, which must be properly formulated, in the present state of the art,

largely by empirical formulation as based on exposure or other tests. A typical example of a satisfactory formulation for either the barrier coating 13, or permissively also for the protective coating 15, of the lacquer type, as employed in conjunction with a lacquer finishing scheme is as follows:

While the foregoing type of formulation is basically for use with lacquer finishing or top coating systems it may also be employed with satisfactory results with air curing (non-baking) enamels such as those of the alkyd or of the epoxy type. In the above formulation the zinc chromate may be replaced by zinc, cadmium, barium, or strontium chromate, molybdate, selenate or vanadate. Preferred substitutions forthe zinc yellow are however either zinc strontium or barium chromate or molybdate. Since a relatively high proportion of such conventional pigmentation best minimizes or prevents any adverse chemical attack by the metalloid constituent upon the light alloy components, even when in direct contact, without employment of the barrier film, and without preventing or interfering with the protective polarization required for the heavy metal components of the structural system.

Where such a general purpose epoxy primer or barrier is desired, as for use with an all epoxy system, the formulation of Table II may be preferably modified by replacement of 50 to 75% of the antimony trioxide content, by weight, with an equal weight of strontium or zinc chromate, or of zinc, barium, orstrontium molybdate. The exact composition is subject to considerable variation provided that the proportion of metalloid constituent retained unsubstituted can be tolerated by the more susceptible light alloy.

Where the metal surface will not hold a paint finish or if a surface lubricant action is required, the finelydivided metalloid constituent is preferably dispersed in a lubricant-type vehicle. Suc-h vehicles-include oils, greases, waxes, pastes, resins and gels. The viscosity of the vehicle should be high enough to keep the finely-divided metalloid fairly evenly dispersed. The amount of metalloid incorporated into said vehicle is dependent upon the severity of the corrosive conditions. 25% by weight of metalloid appears to be optimum for a severely corrosive environment. Such employment in either a grease or dry film or resin lubricant composition will minimize the corrosive attack resulting from the inclusion of conductive solid lubricants such as graphite or molybdenum disulfide.

Again referring to FIG. 2, if the metal 17 is a light metal as previously described, a single protective coating containing the metalloid constituent can be employed if there is a suflicient proportion of a conventional anionic constituent, such as zinc yellow, strontium chromate, or of strontium or barium molybdate is included in the coating 15. In general, for commercial high strength aluminum, magnesium or titanium alloys, a general weight proportion of 1 to 1 as between, for example, antimony oxide and zinc yellow or strontium chromate gives exexcellent results. For commercial magnesium cast or wrought alloys, this ratio is preferably 1 to 2 or 1 to 3.

Thus, it should be noted that while in the application of a metalloid compound coating on a light metal may normally require an additional barrier layer, if a sufficient proportion of the conventional anionic corrosion inhibitor previously described is incorporated with the metalloid in coating 15, the need of the barrier coating 13 is eliminated.

Examples of a single protective coating 15 incorporating the metalloids and the anionic inhibitors are as follows:

TABLE IV PRIMER COATING, NITROCELLULOSE MODIFIED ALKYD Min. Max.

Solids Content:

Total solids (percent by weight of primer) 45 Pigment content (percent by weight of total solids) 47 53 Pigment Content:

*Zinc yellow (percent by weight) 33 Antimony trioxidc (percent by weight) 17 25 Siliceous extender 50 N on Volatile Vehicle Content:

Nitroccllul ose 23 27 Alkyd resin (Phthalio Anhydride-Linseed Oil) 73 77 Volatile Content:

Esters and ketones (total) Medium or high boiling 20 Alcohols. 15 22 Medium or high borlmg 10 Hydrocarbons *Also may substitute, for example, strontium chromate, strontium molybdate, and barium molybdate in same proportion.

Ol Strontium molyhdate 250 Antunony trioxide. 100 Talc (line) extender... 50 Alkyd resin (60% solu n) 336 Maleic anhydride 1 1 Dispersion phenolic resin (50% solution) 84 84 Lead uaphthcnate (24% metal) 3. 7 3. 7 Cobalt naphtllenate (6% metal) 0. 9 0. 9 10% aluminum gel 34 38 Antiskinning agent 0. 9 0. 0 Xylene 174 188 0} may substitute zinc yellow or 9 Strontium chroinate respectively. Satisfactory formulations are not however restricted to the above exact proportions or formulation minor constituents.

Another modification of the invention is shown in FIG. 3, wherein the metal 17 to be protected, e.g., steel or iron, has a plating 19 of antimony or arsenic. The application of a coat of conventional corrosion-resistant paint or primer 21, in which the inhibitor is a slightly soluble chromate, selenate or vanadate, directly to the plated steel or iron gives unexpectedly superior protection because of the previously described synergistic effect of the metalloid upon the inhibitor.

The most serious corrosion problems arise where dissimilar metals are coupled, bonded or otherwise in contact with one another. This so-called galvanic corrosion, which is in addition to environmental corrosion, has been attributed to electropotential differences between the metals. The farther apart the dissimilar metals are in the electromotive series, theoretically the greater the rate of corrosion. This has been shown to be untrue or, at least, not the whole story. Of the common structural metals, a magnesium-iron couple gives rise to the fastest rate of galvanic corrosion. The rate of corrosion is slower for a tin-magnesium couple, although tin is farther removed than iron from magnesium in the series, and the rate is still farther removed. It appears that the chemical corrosion mechanisms and film forming reactions peculiar to the individual metals and their combination control the rate of galvanic corrosion between dissimilar metals, rather than differences in electropotential.

FIG. 4 illustrates the general application of this invention to coupled dissimilar metals 11 and 23 of any type, whether ferrous or nonferrous. All surfaces requiring protection from corrosion are coated with paint or primer barrier layer 13 which can be chemically inert or, preferably, incorporate a slightly soluble corrosion inhibitor. Where the dissimilar metals border each other, a metalloid-containing protective coat 15 need be applied to only one of the facing surfaces, the corrosion resistant properties of the metalloid being effective on both surfaces.

Another embodiment of the invention, as applied to coupled dissimilar metals, is shown in FIG. 5, where 17 is a ferrous structural metal. In such a case protective coat 15 or metalloid plating 19 can be applied directly to the ferrous member. Barrier layer 13 must be applied to dissimilar nonferrous metal 11, however and should be chemically inert thereto. Again, the single metalloidcontaining protective layer 15 or 19 suffices for both facing surfaces.

Where a barrier layer-metalloid combination is used, as taught by this invention, galvanic corrosion is completely controlled, even when the couple is magnesiumiron and the environment is saline. Prior art, such as tinplating, merely reduces the rate of galvanic corrosion as compared to a coupling of the bare metals.

Technological developments in advanced types of aircraft and airborne missiles have increased the extent of applications requiring the so-called dry film lubricants. These conventionally are dispersions of lubricative pigments, such as finely divided natural graphite or of natural molybdenum disulfide, or molybdenite, in a solvent-thermosetting resin vehicle, applied as a thin coating to the frictional area in question and bake-cured to produce a lasting friction reducing surface. The two criteria commonly applied are the resultant reduction in the coefiicient of friction and the persistence or wear resistance of the coating so applied. The initial wear resistance depends largely on the surface adhesion of the applied coating and is generally superior on chemically treated and converted surfaces, rather than on bare metallic protective plated coatings. However, either on heavy metals, such as ferrous alloys, or on light alloys such as those of aluminum, the inclusion of the conductive pigments, such as graphite or of molybdenum disulfide tends to destroy the dielectric protection of the resin constituent and on exposure to atmospheric storage or use the coating and the substrate deteriorate due to corrosion, because of the limited protection afforded by such a system, and this limits the usefulness and serviceability of such applications. The present disclosure has very greatly improved the performance and reliability of such systems by providing for the additional inclusion of a metalloid. or a metalloid compound to hinder or prevent this deterioration without any sacrifice of lubricant efficiency in performance. A specific embodiment of this application is a coating, as applied to phosphated steel sufaces consisting of:

Thinned to the required application extent with dioxane. Bake cured, after air drying, at 325 to 375 F.

The exact proportions are not, however, particularly critical as to effective performance. For application to light alloys, such as a chromate surface treated aluminum alloy in place of the 20% antimony oxide constituent there should be substituted 10% of the same plus 10% of a conventional inhibitive pigment, such as strontium chromate or molybdate.

It is apparent that this invention teaches the utilization of the metalloids antimony and arsenic and certain of their compounds, as above described, to control corrosion of all metals, not only steel and iron. The incorporation of said metalloids into an air-drying lacquer system makes protection of the structural metals both convenient and economical. In addition, the teaching of the synergistic effect of the metalloids upon conventional chromate, selenate and. vanadate corrosion inhibitors affords greater corrosion control than heretofore possible.

It is understood that the proportions listed in the various examples recited in this application are not to be construed as limiting the invention.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

I claim:

1. The combination of a heavy metal and a light metal in a single structural system having contiguous surfaces,

(a) said light metal selected from a group consisting of aluminum, magnesium and titanium and alloys thereof,

(b) a first coating including a metalloid constituent selected from the group consisting of antimony and arsenic applied to the surface of the heavy metal, and

(c) a barrier layer second coating on the surface of such light metal and compatible therewith for insulating the light metal from chemical reaction with the first coating,

(d) at least one of the coatings containing a corrosion inhibitor selected from the group consisting of a low solubility chromate, molybdate, selenate and vanadate,

(e) said coatings being in contact with each other and with the respective metals;

whereby said first coating protects the heavy metal from external corrosive agents and said second coating protects the light metal from direct reaction by said first coating.

2. The combination of claim 1 wherein a said first coating contains the corrosion inhibitor.

3. The combination of a heavy metal and a light metal in a single structural system having contiguous surfaces,

(a) said light metal selected from a group consisting of aluminum, magnesium and titanium and alloys thereof,

(b) said metals being separated and in contact with a composition including a metalloid constituent selected from the group consisting of antimony and arsenic and a substantial amount of a corrosion inhibitor selected from the group consisting of a low solubility chromate, molybdate, selenate and vanadate,

whereby said metalloid constituent and the corrosion inhibitor act in a synergistic manner, increasing the solubility and anticorrosive activity of the corrosion inhibitor at the metal boundary.

4. In combination with (a) a heavy metal selected from a group consisting of car hon and alloy steel, iron and. those of this group haying electroplated surfaces,

(b) a coating applied to the heavy metal including a metalloid constituent of antimony and arsenic,

(c) the coating containing a corrosion inhibitor selected from the group consisting of low solubility chromates, molybdates, selenates and vanadates whereby said metalloid constituent and the corrosion inhibitor act in a synergistic manner, increasingthe solubility and anticorrosive activity of the corrosion inhibitor at the metal boundary.

References Cited by the Examiner UNITED STATES PATENTS 8,275 8/1851 Wetterstedt 106-15 58,458 10/1866 Morse 10615 59,568 11/1866 Eames 106-15 2,301,983 11/1942 Tanner 117-127 ALEXANDER WYMAN, Primary Examiner.

MORRIS SUSSMAN, EARL M. BERGERT,

Examiners. 

1. THE COMBINATION OF A HEAVY METAL AND A LIGHT METAL IN A SINGLE STRUCTURAL SYSTEM HAVING CONTIGUOUS SURFACES, (A) SAID LIGHT METAL SELECTED FROM A GROUP CONSISTING OF ALUMINUM, MAGNESIUM AND TITANIUM AND ALLOYS THEREOF, (B) A FIRST COATING INCLUDING A METALLOID CONSTITUENT SELECTED FROM THE GROUP CONSISTING OF ANTIMONY AND ARSENIC APPLIED TO THE SURFACE OF THE HEAVY METAL, AND (C) A BARRIER LAYER SECOND COATING ON THE SURFACE OF SUCH LIGHT METAL AND COMPATIBLE THEREWITH FOR INSULATING THE LIGHT METAL FROM CHEMICAL REACTION WITH THE FIRST COATING, (D) AT LEAST ONE OF THE COATINGS CONTAINING A CORROSION INHIBITOR SELECTED FROM THE GROUP CONSISTING OF A LOW SOLUBILITY CHROMATE, MOLYBDATE, SELENATE AND VANADATE, (E) SAID COATINGS BEING IN CONTACT WITH EACH OTHER AND WITH THE RESPECTIVE METALS; WHEREBY SAID FIRST COATING PROTECTS THE HEAVY METAL FROM EXTERNAL CORROSIVE AGENTS AND SAID SECOND COATING PROTECTS THE LIGHT METAL FROM DIRECT REACTION BY SAID FIRST COATING. 